DESCRIPTION: Understanding the cellular mechanisms that underlie attention is crucial for developing effective treatments for the many psychiatric and learning disorders that affect attention. We have developed experimental protocols in the barn owl that allow us to study the cellular basis of a fundamental component of attention, stimulus selection, in quantitative detail. The aim of the proposed research is to analyze characteristics of bottom-up (automatic) and top-down stimulus selection at the neuronal level and to elucidate mechanisms that underlie these processes. The properties of bottom-up and top-down stimulus selection will be studied in the optic tectum, a structure known to be involved in spatial attention. The effects of multiple, simultaneous sensory stimuli on neuronal responses will be measured as the salience and location of competing stimuli are varied parametrically. Top-down signals (originating in the forebrain) that bias stimulus selection will be activated by electrical microstimulation. Finally, the respective roles of specialized cholinergic and GABAergic nuclei in bottom-up and top-down stimulus selection will be explored with electrophysiology and pharmacological inactivation experiments. The mechanisms by which the brain selects stimuli for attention are not known. In addition, the effects of cholinergic input on information processing in the brain are not understood. Many debilitating conditions (e.g., Schizophrenia, Autism, ADHD) include dysfunctions of stimulus selection, and others (e.g., Alzheimer's, Parkinson's, Down's) are associated with dysregulation of cholinergic transmission. An understanding of the cellular mechanisms of stimulus selection and the function of cholinergic circuits is crucial for developing treatments that can mitigate or remediate the devastating effects of these conditions.